1. Field of the Invention
The present invention relates to a catalytic combustion apparatus, and it particularly relates to a catalytic combustion apparatus using a self heating type catalyst.
2. Description of the Prior Art
Catalyst combustion which is surface combustion on the catalyst differs completely in combustion process from usual vapor combustion. Thus, the catalyst combustion has many advantageous features. For instance, as a typical feature, since a catalyst temperature can be suppressed down to 1000.degree. C. or lower, generation of thermal NO.sub.x (nitrogen oxides) can be suppressed to a great degree. Moreover, due to the surface combustion on the catalyst combustion, a combustor itself becomes a radiation unit, so that pleasant radiation heating can be realized when the combustor is employed as a room heater. Moreover, there is little concern for a flame since a combustion temperature is low. Moreover, since there is no need for a combustion chamber necessary for the conventional combustor, the combustor becomes compact-sized.
In order to perform a desirable catalytic combustion on the catalyst, it is necessary that reaction gas quantity is proper corresponding to a catalyst volume and catalyst area, and that the catalyst is kept above a temperature by which the catalyst is sufficiently active against the reaction gas. Usually, such a temperature is approximately 500.degree. C. minimum under the combustion in combination of a precious metal catalyst such as Pt (platinum) and Pd (palladium) and a hydro carbon or the like. That is to say, sufficient reaction cannot be performed unless the temperature becomes approximately 500 .degree. C. or more, and if the temperature is below 500.degree. C., produced is unburnt gas including carbon monoxides (CO) harmful to human body and unburnt hydro carbon attributable to unpleasant smell.
In the combustion apparatus such as a stove or the like, the catalyst temperature is as low as an ambient temperature at the time of ignition. Under such condition, the reaction gas is not ignited and the unburnt gas is discharged. Thus, an ignition operation is necessitated to preheat the catalyst in advance and supply the reaction gas thereafter.
Conventionally, as a general preheating method, there is available a method by which a preheating burner or a preheating heater is provided in an upstream side of catalyst mass to primarily heat the air and then feed the primarily heated air to the catalyst mass so as to preheat the catalyst mass. However, many problems are caused thereby as follows.
Namely, in both the preheating burner and the preheating heater, it is not efficient that the catalyst is preheated indirectly through the air preheated. In this case, quite a bit of volume of air besides the catalyst mass has to be heated, and the surrounding parts such as an air passage are heated concurrently and unnecessarily, thus wasting a great deal of energy and time. Moreover, temperature is distributed unevenly to the air to be heated, and the catalyst mass temperature thus becomes unevenly distributed As a result, at the time of ignition a part of the reaction gas passing through a low-temperature portion of the catalyst mass passes through as being unreacted, thus causing a problem of producing odor and white flame. Furthermore, a space for installing the preheating heater has to be provided, thus causing a problem where the apparatus becomes rather bulky and the extra cost therefor is necessary. When a kerosene burner is used as the preheating burner, further added time will be required since there will be needed a further rise time such as for preheating a carbureter of the kerosene burner itself. In that case, the preheating burner performs a vapor combustion in which NO.sub.x is naturally produced. Moreover, carbon is a catalyst poison. Even a small quantity of soot (carbon) may cause the catalyst to deteriorate when the soot is absorbed into the catalyst.
When using the preheating heater, a large-volume heater for heating the air is required. Accompanied by the large-volume heater, a large-volume relay circuit and thick lead wires and so on will be also necessitated, thus increasing otherwise unnecessary electric parts so as to cause a problem where the initial cost such as the cost for producing a finished product and the operational cost such as electricity consumption increase.
Now, in order to have a pleasant heating without an on-off switching operation in a combustion apparatus such as a stove, it is required to have wide range of heating capacity to vary. It is to be noted that catalyst temperature varies according to the heating capacity, that is, combustion quantity.
FIG. 1 shows a typical performance characteristic of the conventional catalytic combustion apparatus. The catalyst temperature rises as the quantity of reaction gas increases up to temperature T2 where the reaction gas is flashed back to the air-fuel mixture (reaction gas) in the upstream side. The catalyst temperature declines as the quantity of reaction gas decreases to temperature T1 where a CO density of the unburnt gas goes over the allowable value. Both T1 and T2 indicate the limit of combustion. Namely, quantity G1 at temperature T1 is the lower limit of combustion quantity whereas G2 at T2 the upper limit. Under normal circumstances, the ratio of quantity G1 of reaction gas over quantity G2 of reaction gas is said to be 1:3 maximum.
FIG. 2 shows a relation between the heating capacity and the catalyst temperature. Heating capacity Q1, Q2 corresponding to reaction gas quantity G1, G2 respectively are the lower limit and upper limit of the heating capacity, respectively. The variable range of heating capacity is 1:3 maximum. Recently, the heat insulating capacity in regular houses has been significantly improved. In this connection, it is not necessary to have a large-scale heating capacity once the temperature reaches the set-up temperature. For this reason, not the conventional ratio of 1:3 as the heating capacity variable range but something of 1:10 is required in order to achieve pleasant heating without an on-off switching operation. In the conventional catalytic combustion apparatus, the on-off switching operation is of course needed, but this on-off switching operation causes unpleasantness due to the change of room temperature accompanied by the on-off switching. Moreover, the on-off switching operation consumes otherwise unnecessary electricity, thus creating a problem in view of conservation of energy. Moreover, there is a problem concerning heat shock which damages material as a result of an extreme change in temperature in the course of thermal expansion. This problem occurs very frequently because the catalyst temperature varies corresponding to the heating capacity. The on-off switching operation accelerates the damage degree of heat shock so as to shorten the life-span of the catalyst itself.
Distinguished from the conventional combustor employing vapor reaction, the catalytic temperature becomes very high by the catalytic surface reaction in the catalytic combustion apparatus utilizing a contact catalytic reaction. Thus, when used for a long period of time, the catalyst becomes deteriorated. Sintering is the typical symptom to indicate the deterioration of catalyst. Sintering is such that active ingredients such as Pt and Pd which are evenly distributed as small particles on the catalyst mass are combined and thus the surface area of the active ingredients can not be secured, so that the reaction activity of the catalyst as a whole deteriorates. Another symptom is that the reaction activity of the catalyst deteriorates when the active ingredients evaporate. When the reaction activity of the catalyst deteriorates, the catalyst temperature declines since sufficient reaction does not occur. When the catalyst temperature declines, the reaction activity of the catalyst is further suppressed thus causing a vicious circle. In this case where the deteriorated catalyst is used, the unburnt gas is mixed into the catalyst mass so as to cause a problem where odor is produced, the heat efficiency is decreased and a poisonous gas is generated. Moreover, in other cases, a problem of catalyst vibration phenomenon may occur that the catalyst temperature vibrates and fluctuates severely. Thus, when such a catalytic combustion apparatus is applied to a heating apparatus, it must be considered to provide catalyst deterioration detecting means. In the conventional practice, there is considered a function by which the density of poisonous gas is detected, for example, by an unburnt gas detecting sensor such as a density detecting sensor of CO. Moreover, there is considered a function by which the catalyst temperature is detected in a manner that the catalyst temperature drops below the predetermined temperature calculated on the basis of a functional relation between the catalyst temperature and the reaction gas quantity to define deterioration of the catalyst so that the combustion operation is terminated. However, in this case, the deterioration of the catalyst is detected and the combustion operation is terminated only after the poisonous gas is already produced and the catalyst temperature is already dropped, so that the generation of unburnt gas is already in existence.
As a conventional method to start the catalytic combustion, there is available a method in which the air is preheated by the preheating burner or heater provided in the upstream side of catalyst mass and then the preheated air is supplied to the catalyst mass so as to preheat the catalyst mass. However, the method has following drawbacks.
The catalyst is indirectly preheated by heating the air as heating medium, thus causing a great deal of energy loss and consuming much time. Furthermore, unevenness of temperature in the preheated air causes non-uniform temperature distribution in the catalyst mass, thus causing odor and white flame accompanied along with the unburnt gas generated at the time of ignition. Moreover, there is a disadvantage in which the apparatus is of rather bulky size and requires an extra cost for providing a space for the preheating burner or heater.
When a kerosene burner is used as the preheating burner, a rise time for the preheating burner itself, such as the preheating time for the carbureter is additionally required. Moreover, the preheating burner performs vapor combustion which is of course accompanied by occurrence of NO.sub.x. The carbon is a kind of catalyst poison. Therefore, even a small quantity of the carbon (soot) may deteriorate performance of catalyst, when the soot is absorbed to the catalyst.
Since a sufficient variable range for capacity in the conventional catalytic combustion apparatus is not provided, the on-off operation takes place to cause unpleasantness, unnecessary combustion, and deterioration of the life span of catalyst due to the heat shock.
Furthermore, in the conventional practice, the deterioration of the catalyst is detected and the combustion operation is terminated only after the poisonous gas is already produced and the catalyst temperature is already dropped, so that the generation of unburnt gas is already in existence by the time that the deterioration of catalyst is detected, thus causing odor, deterioration of heat efficiency, generation of poisonous gas, and temperature fluctuation of the catalyst.